Grain boundary cohesion and segregation in ultrahigh strength alloy steels

ntergranular segregation and its influence on grain boundary (GB) cohesion are measured in ultrahigh-strength (UHS) martensitic alloy steels involving both model compositions and alloy prototypes, employing Scanning Auger Microanalysis of in situ fracture surfaces. The kinetics of phosphorous and sulfur segregation to prior austenite grain boundaries during secondary hardening of high Co-Ni alloy steels are consistent with bulk diffusivities and segregation energies determined in previous studies of simple model alloys. In contrast to previous studies of GB fracture strength requiring notch geometries to achieve intergranular fracture, the extreme strength levels of the current alloys allow precise measurement of fracture strength ;A fractographic study of intergranular hydrogen embrittlement resistance is undertaken in rapidly solidified model NiMo steels in which P and S impurities are gettered, and which contain excess C relative to M;In prototype alloys designed to provide sufficiently fast alloy carbide precipitation to achieve desired strength prior to significant segregation of P and S, it is found that intergranular fracture can occur from depletion of cohesion-enhancing C at grain boundaries as a result of alloy carbide precipitation in the bulk. A failure map correlating the fracture strength to yield strength ratio...